The recent outbreaks of West Nile fever and Chikungunya fever in North America created a new challenge for the public health system in the United States. The lack of vaccines and treatment options for these diseases raises the importance of personal protection against the mosquito vectors. N,N-diethyl-meta-toluamide (DEET) is the most effective mosquito repellent currently on the market. However, decreasing consumer acceptance of DEET has led to a search for new types of repellents. Serendipitously, we screened several isolates of entomopathogenic bacteria for adulticidal activity and found potent mosquito repellent activity in an extract from Xenorhabdus innexi. Female Aedes aegypti died of desiccation rather than accept sugar water mixed with the bacterial extract. Preliminary data indicate that the active compound is a small peptide and the purified peptide is water soluble and retains repellent activity for at least several weeks. Three specific aims are proposed to further our understanding of the molecule. Aim 1 will use nuclear magnetic resonance (NMR) spectroscopy methods to resolve the structure of the purified repellent peptide. Aim 2 will identify the biosynthetic gene cluster involved in the repellent peptide production in X. innexi, which will enable searches for similar gene clusters in other bacterial species as well as mutagenic approaches for structure/function analyses. Aim 3 will further characterize the novel repellent compound. Here, the efficacy and longevity of the repellent activity will be quantified against several key vector mosquito species. In addition, cell viabilit assays will be performed against several mammalian cell lines to examine the toxicity profile of the repellent peptide. The information generated in this study will contribute to our understanding of a new category of mosquito repellents and may ultimately result in new products for protection against mosquito vectors.